Immunoassay methods have been widely used in clinical tests of serum, plasma, urine, feces, spinal fluid and the like. In recent years, owing to the capability to carry out simple and rapid measurements, automatic analyzers carrying out procedures during from a reaction to a measurement collectively and automatically have been widely used.
The immunoassay methods are known to be measurement methods utilizing antigen-antibody reactions and showing high specificity. However, this method had a problem that non-specific reactions such as a false positive reaction and a false negative reaction may occur depending on samples. For example, factors which recognize and react with the antibody may be present in samples, and in such a case, even though an antigen to be measured is not present in the sample, the measurement gives a positive result. On the other hand, factors which interfere with the antigen-antibody reaction may be present in the sample, and in such a case, even though an antigen to be measured is present in the sample, the measurement gives a negative result. Thus, it had a problem that the measurements give different results from real value.
As a means for suppressing non-specific reactions, it had been known that a human IgM native antibody or a polymer obtained by polymerizing aromatic monomers having sulfone groups or salts thereof was added (see Patent Documents 1 and 2). However, these additives are not sufficient in some cases, and in particular, it was difficult to suppress non-specific reactions in the low concentration range. Further, reagents with improved sensitivity are likely to cause non-specific reactions because the reactivity of the antibodies is improved.